This invention relates to structures that transport liquids by capillary action. More particularly, this invention relates to fibers and personal hygiene absorbent products such as diapers, adult incontinent pads, and feminine napkins, and to the flow, distribution, and acquisition of liquids in the fibers and products.
In the past several years there has been great interest in polymer structures that provide liquid transport and storage.
U.S. Pat. No. 5,200,248 to Thompson et al. issued Apr. 6, 1993 and discloses capillary channel structures such as fibers that include infrastructure capillary channels that store and transport liquid. The Thompson et al. patent disclosed that these capillary channel fibers may be coated with materials that provide an adhesion tension with water of at least 25 dynes/cm. The teachings and especially the definitions in the Thompson et al. patent are hereby incorporated by reference as if fully set forth herein. This specification provides values for fibers shown in examples herein for some of the quantities defined in the Thompson et al. patent.
European patent application No. EP 0 516 730 B1 claims priority from the application that matured into the Thompson et al. patent.
U.S. Pat. No. 5,611,981 to Phillips et al. issued Mar. 18, 1997 discloses spontaneously wettable fibers having a combination of X values and surface contact angles that satisfy conditions for spontaneous wetting. The X factor is defined therein as X=Pw/(4r+(nxcfx80xe2x88x922)D) where Pw is the wetted perimeter of the filament, r is the radius of the circumscribed circle circumscribing the fiber""s cross-section, and D is the minor axis dimension across the fiber""s cross-section. The teachings of and especially the definitions in the Phillips ""981 patent are hereby incorporated herein by reference as if fully set forth herein. This specification discloses values for fibers shown in examples herein for some of the quantities defined in the Phillips ""981 patent.
U.S. Pat. No. 5,268,229 to Phillips et al. issued Dec. 7, 1993 discloses specific xe2x80x9cUxe2x80x9d and xe2x80x9cExe2x80x9d shaped cross-sections of spontaneously wettable fibers with stabilizing legs.
U.S. Pat. No. 5,314,743 discloses non-woven webs made from capillary channel fibers.
U.S. Pat. No. 3,121,040 to Shaw et al. discloses xe2x80x9c+xe2x80x9d and xe2x80x9cYxe2x80x9d shaped polyolefin fibers with arm length/arm width ratios greater than 4. These fibers are so thick and large that they are too stiff for use in consumer disposables. The smallest arm width disclosed in the Shaw et al. patent is about 75 microns.
International patent application PCT/US95/08896 discloses a structure that is capable of transporting liquids by intercapillary action using essentially parallel fibers, and discloses that the driving force on the liquid is directed from the open areas to the closed areas.
U.S. Pat. No. 4,829,761 to Phillips et al. issued May 16, 1989 discloses continuous filament yarns. The teachings of and especially the definitions in that patent are hereby incorporated herein by reference as if fully set forth herein. This specification provides values for fibers shown herein for the specific volume quantity defined in the Phillips ""761 patent. U.S. Pat. No. 4,245,001 to Phillips et al. also discloses the specific volume quantity, and its teachings are also incorporated by reference as if fully set forth herein. The specific volume is defined in the Phillips ""761 patent in units of cubic centimeters per gram as 8.044 divided by the weight of the yarn in grams when the yarn is under a tension of 0.1 grams per denier for a volume of yarn filling an 8.044 cubic centimeter volume. Thus, the specific volume is the volume per gram of material in a volume of space when the fibers of the yarn are pressed against one another in the volume of space and are under a defined tension.
Much of the interest in polymer structures that absorb and transport liquid is because of their applicability in consumer disposable products. The inventors view absorbent cores of modern consumer disposable products including diapers, adult incontinent pads, and feminine napkins, as having three primary functions; acquisition, distribution, and storage of liquids. The distribution function is typically poorly executed with current absorbent core components such as fluff pulp and/or super absorbent polymer. As a consequence, excessive leakage and poor utilization of the absorbent core material relative to the theoretical maximum absorbent capacity of the absorbent core material are problems limiting the performance of these consumer disposables.
Poor distribution occurs because the components of the core are typically good at storing liquids but poor at distributing them. Many attempts have been made in the prior art to solve this problem.
International Patent Application No. WO 95/00093 dated Jan. 5, 1995 discloses a sanitary pad with a liquid directing strip and an absorbent strip positioned under a top sheet.
U.S. Pat. No. 5,342,336 to Meirowitz et al. issued Aug. 30, 1994 and discloses a structure for absorbing and transporting a liquid that includes shaped staple fibers to move liquids more toward the ends of the pad. Typically, staple fibers are less than two inches long.
U.S. Pat. No. 4,324,247 to Aziz issued Apr. 13, 1982 and discloses an absorbent article including a top sheet, an absorbent core, and a perforated thermoplastic film between the top sheet and the absorbent core. The Aziz patent teaches that its structure prevents liquid in the core from flowing out of the absorbent core back to the top sheet when the structure is squeezed.
U.S. Pat. No. 4,321,924 to Ahr issued Mar. 30, 1982 and discloses an absorbent article including a top sheet, a layer of fibers affixed to the inner surface of the top sheet, the fiber layer overlaying an intermediate layer having a multiplicity of tapered capillaries, and an absorbent core. The Ahr patent asserts that the Ahr structure provides improved acquisition and reduced re-wetting.
United Kingdom Patent Application GB 2,225,724A was published Jun. 13, 1990 and discloses an absorbent device that includes a liquid pervious cover sheet, an absorbent core, and a liquid pervious intermediate layer that is between the absorbent core and the cover sheet and that has apertures and contours. This patent application asserts that its structure provide reduced re-wetting.
U.S. patent application Ser. No. 545,450 filed Oct. 19, 1995 discloses an apertured film with cut out portions in the apertured walls to provide spontaneous liquid inversion from the front side of the top sheet to the backside of the top sheet. The teachings of the ""450 application are hereby incorporated by reference as if fully set forth herein and may be used in conjunction with the absorbent product inventions defined herein.
Thus, there is a general ongoing desire in the art to increase the absorbent capacity and the liquid transport capacity of polymer material for various applications. There is a more specific continuing need in the art for a family of acquisition/distribution structures which can better transport and distribute liquids in disposable absorbent products. Accordingly, it is to the provision of such that the present invention is primarily directed.
Further, it is to be understood that the inventors conceive of additional applications relating to the novel transport ability of the basic fiber structures disclosed herein including filtering of liquids and suspensions, horizontal transport of liquids, and vertical transport of liquids.
The invention is a bundle of synthetic fibers for transporting fluids. The bundle comprises at least two fibers that when acting as individual fibers are poor transporters of fluids, yet when in a bundle the fibers provide a bundle that is an excellent transporter of fluids. The bundles are useful in absorbent articles such as diapers, incontinents and feminine hygiene products.
The bundle has a Specific Volume greater than 4.0 cubic centimeters per gram (cc/gm), an average inter-fiber capillary width of from 25 to 400 microns, and a length greater than one centimeter (cm). Preferably, the fluid to be transported is aqueous and the movement of fluid in the bundle is measured according to the following parameters as defined herein: a MPFB/MPFSFgreater than or equal to 3.0, a MPFB greater than or equal to 0.14 cubic centimeters per denier per hour (cc/(den*hr)), a VRB/VRSF greater than or equal to 1.3, and a VRB greater than or equal to 4.0 centimeters (cm).
At least one of the two fibers has a non-round cross-section, a Single Fiber Bulk Factor greater than 4.0, a Specific Capillary Volume less than 2.0 cc/gm or a Specific Capillary Surface Area less than 2000 cc/gm, and more than 70% of intra-fiber channels having a capillary channel width greater than 300 microns. Preferably, the cross-section defines a first arm having a length greater than 40 microns. The lengths of the cross-section of the fibers range up to almost 1000 microns with some of the examples having arm lengths that are between 100 and 400 microns. Preferably, the fibers have a denier (den) between 15 and 250. The cross-section and the surface composition of the non-round fibers preferably satisfy the inequality: (Pxcex3 cos(xcex8a))/d greater than 0.03 dynes/den, wherein P is the perimeter of the cross-section of the fiber, xcex3 is the surface tension of the liquid, (xcex8a) is the advancing contact angle of the liquid measured on a flat surface made from the same material as the fiber and having the same surface treatment and d is the denier of the fiber.
Further, the invention includes the novel spinnerettes used to make the fibers of the bundles. Those spinnerettes are characterized by very large ratios of the length to the width of the aperture of the spinnerette and large absolute lengths of sections of the aperture of the spinnerette. Preferably, the length to width ratios of a section of the spinnerette is greater than 40, more preferably greater than 60, and even more preferably greater than 100. The length to width ratio of individual cross-section segments (e.g., legs, arms) may be between 40 and about 150.
Further, the process of making the fibers of the present invention includes heating the polymer to between 270xc2x0 and 300xc2x0 centigrade and extruding the heated polymer through an aperture having a width of less than 0.12 millimeters (mm) and a total length of at least 50 times the width.
Further, liquid acquisition/distribution structures are included in the invention which comprise a top layer that is permeable to a liquid, a distribution layer, and a resistance layer. The distribution layer comprises a capillary system providing capillary forces on the liquid when the liquid is in contact with the distribution layer tending to transport the liquid parallel to the top layer. The resistance layer has a resistance layer top surface and a resistance layer bottom surface. The resistance layer provides resistance to transmission of the liquid from the resistance layer top surface to the resistance layer bottom surface. An absorbent core may also be added to the structures which may be beneath the resistance layer or partially surrounded by the distribution layer and the resistance layer.